Someone here had theirs stripped and galvanized it looked perfect inside/out
If you dont have a lot of rust inside just sandblast and paint/coat the outside call it a day.
Spray oil or? on the inside if it bugs you.

 

I am just a hobbyist/tinkerer. but before I tried powder coating, I painted everything with a spray gun. I did a frame off and I blasted my frame to white metal with a cheap harbor freight power blaster and had the local shop powder coat it. I went through the same questions you have and I concluded .I won't run it in the rain and the rust probably provides some protection over bare metal. The frame lasted 47 years, has minimal visible inside corrosion and will be treated better in the future. Is there much if any benefit to me worrying about the frame inside considering my future use? I did toy with using 3M cavity wax, but I wasn't sure I could completely coat the frame and if not I may be making the uncoated areas more susceptible to corrosion.

I powder coated everything that could possibly be coated with a cheap Eastwood gun and an old household oven. I even powder coated the heads of the bolts and nuts. You are absolutely right. Once it is powder coated there is no tweaking anything, but the powder coating holds up extremely well. I had a gas leak that had no effect on the coating and I used SS brake lines which I had to work like crazy to get to seal. Ie Brake fluid on many components. No damage. Cleanup is a breeze You just blow the gun out. And it is much cheaper than good quality paint.
I reworked smaller pieces after they were powder coated by blasting the coating off. The coating is extremely hard to remove even with sand blasting. I also found that you cannot use any abrasives on the coating (I.e. rubbing compound) it scratches and I couldn't get the scratches out.

I stayed away from heating the frame excessively. (800F+) No good can come from putting that much heat on the frame. I suspect the residual stress and heat will at least somewhat distort the frame. Galvanizing is a really great option if you want the frame to last forever. I personally believe it depends on how you are going to use the car as to what technique you should use.

 

I know this post was recently 'revived' but I am close to this part of my project and was looking for any alternatives before I move on.

My '72 frame is original and obviously has rust, but nothing to the point of significant repairs. I would like the frame to outlast me, which god willing will double the age of the car. I live in Cleveland and the closest, reasonable dip location I have found is in Nanty Glo, Pennsylvania. Before I rent a truck and/or trailer combo and drive 6 hours round-trip twice with crappy gas mileage and ample tolls, does anyone have a place they would recommend, considering the geography?